Greater volumes of a callosal sub-region terminating in posterior language-related areas predict a stronger degree of language lateralization: A tractography study

Abstract

Language lateralization is the most intriguing trait of functional asymmetry for cognitive functions. Nowadays, ontogenetic determinants of this trait are largely unknown, but there are efforts to find its anatomical correlates. In particular, a white matter interhemispheric connection–the corpus callosum–has been proposed as such. In the present study, we aimed to find the association between the degree of language lateralization and metrics of the callosal sub-regions. We applied a sentence completion fMRI task to measure the degree of language lateralization in a group of healthy participants balanced for handedness. We obtained the volumes and microstructural properties of callosal sub-regions with two tractography techniques, diffusion tensor imaging (DTI) and constrained spherical deconvolution (CSD). The analysis of DTI-based metrics did not reveal any significant associations with language lateralization. In contrast, CSD-based analysis revealed that the volumes of a callosal sub-region terminating in the core posterior language-related areas predict a stronger degree of language lateralization. This finding supports the specific inhibitory model implemented through the callosal fibers projecting into the core posterior language-related areas in the degree of language lateralization, with no relevant contribution of other callosal sub-regions.

The relationship between white matter architecture and language lateralisation in the healthy brain

“…Notably, our shape analysis revealed no significant correlation between shape measures of the callosal tracts and language lateralisation, consistent with Westerhausen et al (2006), who proposed that the microstructural organization, rather than the macrostructure, of the corpus callosum indicates functional language lateralisation. These findings differ from a recent study by Karpychev et al (2022), which found that greater volumes of a posterior corpus callosum subregion were associated with a more substantial degree of language lateralisation (both RLD and LLD). Although subtle or no significant association was observed between long-range association fibres (including the inferior fronto-occipital fasciculus, superior longitudinal fasciculus, and uncinate fasciculus) and LQ in line with previous research (Delgado-Fernández et al, 2020; James et al, 2015; Silva & Citterio, 2017), a few earlier studies have reported conflicting results.…”

“…Several studies have investigated the relationship between characteristics of white matter and HLD, focusing on morphometric features (Josse et al, 2008; Karpychev et al, 2022; Westerhausen et al, 2006). However, no studies have combined tractography shape analysis with functional data in healthy individuals to examine the connection between geometric features of tracts and language lateralisation.…”

“…Most existing studies have used similar tractography methods, and some have reported significant correlations between the fMRI-determined left HLD and FA of the left arcuate fasciculus (James et al, 2015;Perlaki et al, 2013;Powell et al, 2006;Silva & Citterio, 2017) and corpus callosum (Häberling et al, 2011). However, other studies have not reported relationships between the side or extent of HLD and conventional diffusion-based tract characteristics (Karpychev et al, 2022;Vernooij et al, 2007), as well as more sophisticated diffusion MRI measures, such as fibre density crosssection (Verhelst et al, 2021). Inconsistencies between studies may be due to methodological differences, including differences in tractography approaches, study designs, patient characteristics, and sample sizes.…”

Influence of environmental and biological factors on functional connectivity of the superior temporal sulcus in neonates scanned at term age

“…It has been explained by asymmetric callosal fibers favoring transfer toward the left temporal region (Andoh et al, 2015). Karpychev et al (2022) recently bring new argument to the role of the corpus callosum in language lateralization, showing the relation between language lateralization in adults and the volume of the temporal callosal fibers in the splenium measured with constrained spherical deconvolution (CSD). As the left posterior temporal region is notably involved in phonetic coding and intelligibility (Hickok & Poeppel, 2007;Vagharchakian et al, 2012), these results underline the centrality of this region in auditorylinguistic networks from the outset.…”

“…An asymmetric inter-hemispheric between auditory regions through the corpus callopsum has been described in infants (Adibpour et al, 2020) and adults (Andoh et al, 2015; Gotts et al, 2013; Karpychev et al, 2022). In adults, repetitive stimulation of the right auditory cortex with TMS modulates its connectivity with the left contralateral temporal cortex but not the reverse.…”